Equine Cerebellar Abiotrophy

Other Names

Cerebellar ataxia (CA)
Cerebellar cortical abiotrophy (CCA)
Neonatal cerebellar ataxia

What is Cerebellar Abiotrophy in Horses?

Cerebellar abiotrophy (CA) is an inherited neurological condition in which the Purkinje cells of the cerebellum — the large neurons responsible for coordinating movement and balance — undergo progressive degeneration after birth. The disease is most commonly recognised in the Arabian breed and related part-bred Arabians, though it has been reported rarely in other breeds.

The cerebellum is the region of the brain responsible for fine motor control, balance, and the coordination of voluntary movement. When its cells die, horses lose the ability to regulate movement accurately — resulting in a characteristic combination of incoordination, intention tremor, and exaggerated, high-stepping limb action that is often described as a “goose-stepping” gait.

Cerebellar abiotrophy is present from birth as a genetic defect, though clinical signs may not always be apparent immediately. In some foals, signs are obvious within the first few weeks of life; in others, they may become apparent more gradually over the first few months. The condition does not cause pain, but severely affected foals are at high risk of injury from falls.

Causes

CA in Arabian horses is caused by a point mutation in the TOE1 (Target of EGR1) gene, which also affects the adjacent MUTYH (Mut Y Homolog) gene. This mutation is inherited in an autosomal recessive pattern, meaning:

A horse must inherit two copies of the mutant gene (one from each parent) to be clinically affected.
Horses inheriting only one copy are carriers — they appear clinically normal but can pass the mutation to offspring.
When two carriers are bred together, there is a 25% chance with each foal of producing an affected individual.

Carrier horses are widespread within the Arabian population due to historical use of influential sires that were unknowingly carriers before the genetic test became available. Certain bloodlines carry a higher frequency of the mutation than others.

Symptoms and Signs

The classic clinical presentation includes:

Ataxia (incoordination): Wide-based stance, swaying, stumbling, and difficulty maintaining balance — particularly noticeable on turns or when navigating uneven ground.
Head tremor: A characteristic intention tremor of the head — trembling that occurs when the horse reaches for feed or water and is attempting a purposeful movement.
Dysmetria: Exaggerated, high-stepping limb action (hypermetria) where the horse lifts its legs much higher than necessary. The gait can look almost theatrical.
Inability to track feet accurately: The horse may overshoot when placing its feet, resulting in a choppy, uncoordinated walk.
Falls: Severely affected foals may be unable to stand without assistance or fall frequently, placing them at serious risk of injury.
Normal mentation: Unlike many neurological conditions, affected horses are mentally alert and aware of their surroundings — the deficit is purely motor coordination.

Signs tend to remain stable rather than progressively worsening once the initial degeneration has occurred, though they do not improve. Mildly affected horses may live relatively normal lives if managed carefully in a safe environment.

Diagnosis

Clinical examination: The characteristic combination of head tremor, hypermetric gait, and ataxia in an Arabian foal is highly suggestive of CA. A thorough neurological examination by a veterinarian is essential to characterise the deficits and rule out other causes of ataxia.
Genetic testing: A definitive diagnosis can be obtained through a DNA test that detects the specific genetic mutation. The test requires a blood sample or hair with intact follicles (30–40 hairs). Results identify horses as normal (N/N), carrier (N/CA), or affected (CA/CA). This test is available through several equine genetic laboratories including the UC Davis Veterinary Genetics Laboratory.
Exclusion of other causes: Other conditions causing ataxia in foals, such as wobbler syndrome (cervical vertebral stenotic myelopathy), equine protozoal myeloencephalitis (EPM), and equine herpesvirus myeloencephalopathy (EHM), must be excluded. These can be differentiated through appropriate diagnostic testing including CSF analysis and spinal imaging.

Treatment

There is currently no treatment for cerebellar abiotrophy — the neuronal loss is irreversible and cannot be halted or reversed. Management options depend on the severity of clinical signs:

Mild cases: Horses with mild signs may live as companion animals (pasture pets) with appropriate environmental modifications — safe, level paddocks with no hazards, soft footing to reduce injury risk from falls, and companionship from calm, compatible horses.
Severe cases: Foals that cannot stand safely, fall frequently, or sustain repeated injuries have a very poor quality of life. In these cases, humane euthanasia is generally the most appropriate course of action, and most veterinary guidelines support this recommendation.
Safety considerations: Even mildly affected horses should not be ridden or used for any activity that places the horse at risk of falls, as this poses a significant safety risk to both horse and handler.

Prevention

CA is entirely preventable through responsible breeding practices:

Test all Arabian horses (and part-bred Arabians) intended for breeding using the available genetic test before any breeding decisions are made.
Never breed two carriers (N/CA) together — this risks producing affected offspring.
Carrier horses can be bred safely to a horse confirmed to be normal (N/N) — no affected foals will result from this pairing, though approximately 50% of offspring will be carriers.
Review pedigrees carefully — some bloodlines carry a higher prevalence of the mutation.
The Arabian Horse Association and many breed registries actively promote CA testing as part of responsible breeding programmes.

Prognosis

The prognosis for affected horses is poor to very poor for any working or ridden life. Mildly affected horses may survive for years as companion animals in a safe managed environment, but they will always carry the neurological deficits and pose safety risks. Severely affected foals rarely survive long-term. There is no reported improvement over time — the underlying neuronal loss is permanent. Affected horses should never be bred, as all offspring will carry at least one copy of the mutated gene.

When to Call the Vet

If an Arabian or part-bred Arabian foal shows any signs of unsteadiness, head tremor, exaggerated limb movement, or falls without obvious reason, contact your veterinarian immediately. Early veterinary assessment is important both for welfare reasons and to confirm the diagnosis via genetic testing. For related reading on neurological conditions, see our entry on Equine Myeloencephalopathy. For more on the Arabian breed, visit our breed profile. The British Equine Veterinary Association can assist in locating an equine neurologist.

Frequently Asked Questions

Is cerebellar abiotrophy painful for horses?

CA does not cause pain. The condition affects motor coordination rather than sensory pathways, so affected horses do not experience the discomfort associated with musculoskeletal disease or nerve pain. However, the repeated falls and inability to move and balance normally that come with severe cases represent a significant welfare concern — not because of pain, but because of the stress and injury risk associated with loss of balance and coordination.

Can a horse with CA lead a normal life?

This depends entirely on the severity of the signs. Mildly affected horses with head tremor and mild incoordination can live as companion animals and may have good quality of life in a carefully managed environment. They should not be ridden or used for any ridden work. Severely affected foals that fall repeatedly and cannot stand safely have a very poor quality of life and euthanasia is generally recommended on welfare grounds.

Should I test my Arabian horse even if it seems normal?

Yes, absolutely. Carrier horses are clinically normal and indistinguishable from non-carriers without genetic testing. Given that carriers can unknowingly pass the mutation to offspring and produce affected foals when bred to another carrier, genetic testing of all Arabians intended for breeding is strongly recommended. Testing is simple, inexpensive, and provides certainty that protects the welfare of future foals.

Are there other breeds affected by cerebellar abiotrophy?

Cerebellar abiotrophy associated with the same TOE1 mutation is primarily an Arabian breed condition. However, similar (though genetically distinct) cerebellar degenerative conditions have been described in other breeds including the Gotland Pony, Oldenburg, and various other breeds. These cases are rarer and caused by different genetic mutations. Any breed of foal showing signs of cerebellar dysfunction should receive a full veterinary neurological assessment.